Advances in wireless technologies have propelled a migration in features and services provided to the end user. Network operators may however need to support multiple and perhaps migratory technologies with limited spectrum. Therefore, radio resource management techniques that improve spectral efficiency and/or system capacity are always of interest to network operators.
Higher spectral efficiency and/or voice capacity can be achieved in the Global System for Mobile Communication (GSM) Enhanced Data rates for Global Evolution (EDGE) Radio Access Network (GERAN) through tight frequency reuse (e.g., 1/3 or 1/1 reuse). Current GSM deployments employ techniques such as frequency hopping in order to combat the effects of fading and interference. The performance improvement achieved through frequency hopping for voice users at the link and system level directly translates into higher capacity.
On a GSM full rate traffic channel, 20 ms (milli-second) speech frames are convolutionally encoded and diagonally interleaved over a sequence of 8 bursts in a time slot. In the case of a half rate channel, speech is coded and diagonally interleaved over a sequence of 4 alternate bursts in a time slot. Frequency hopping is carried out burst by burst in order to mitigate the effects of slow fading and interference. It provides the following benefits: fading diversity, interferer diversity, and interference averaging.
In practical systems, the frequency hopping is typically non-ideal and the benefits of fading and interferer diversity are not fully realized. With respect to frequency hopping techniques, GSM specifies cyclic frequency hopping and pseudo-random frequency hopping (e.g., see 3GPP TS 45.002, “3rd Generation Partnership Project; Technical Specification Group GERAN; Digital Cellular telecommunications System (Phase 2+); Multiplexing and Multiple Access on the Radio Path (Release 4)”). If the number of frequencies is sufficient, then cyclic hopping provides full fading diversity. (As referred to herein, full fading diversity is where every burst within the interleaving depth of a speech frame experiences an independent fading state. This is possible only if the number of frequencies is greater than the number of bursts over which a speech frame is interleaved and the frequencies are sufficiently separated from each other.) However, cyclic hopping does not provide the benefits of interferer diversity and interference averaging. The pseudo-random frequency hopping algorithm specified in GSM provides interferer diversity and achieves long-term interference averaging but does not guarantee fading diversity (i.e., no frequency repetitions) within the interleaving depth of a speech frame.
With respect to GSM pseudo-random frequency hopping, if a large amount of spectrum is allocated, then there are many frequencies over which users can hop and repeated frequencies over a short interval are not common. However, in limited spectrum scenarios where the number of frequencies are smaller than the number of bursts over the interleaving depth (40 ms in the speech case), frequency repetitions always occur. This is illustrated in FIG. 1 on a full rate traffic channel. For full-rate voice users, eight bursts are transmitted over pseudo-randomly generated frequencies (it is assumed for this example that there are eight frequencies to select from: f0 to f7). As can be observed from FIG. 1, coded speech frame 1 encounters frequency, f4, on 3 out of the 8 bursts that it is interleaved across. This implies that speech frame 1 experiences only 6 out of 8 possible independent fading states (assuming there is sufficient separation between each of the frequencies). Similarly, it can be observed for speech frame 2 that frequencies, f2, f4 and f5 are repeated two times each on the 8 bursts over which coded speech frame 2 is interleaved. In this case, speech frame 2 experiences only 5 out of 8 possible independent fading states. In other words, the GSM pseudo-random frequency hopping algorithm does not maximize the number of unique frequencies (or independent fading states) in this case. This has consequences for low mobility users where the fading tends to be strongly correlated for time duration in excess of the interleaving depth of a speech frame. In this case, users may hop to the same frequency multiple times, experiencing similar channel fading conditions each time. With typical channel coding schemes employed for speech traffic channels and control signaling channels, increased correlation within the interleaving depth can lead to degradation in error performance.